Method for surface cooling steel slabs to prevent surface cracking, and steel slabs made by that method

ABSTRACT

A method is provided for the continuous casting, cutting, and continued heat treatment of steel slabs, particularly those having cracking-prone alloy formulations, without requiring the use of water spray quench cooling equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

REFERENCE TO A “MICROFICHE APPENDIX”

Not Applicable.

BACKGROUND OF THE DISCLOSURE

1. The Technical Field

The present invention relates to methods for continuous casting of steelslabs.

2. The Prior Art

In conventional continuous casting mills with direct hot charging, steelin a caster assembly is cast into a continuous strand, and passesthrough a strand containment apparatus in which the steel surface iscooled and the strand changes direction from the vertical to thehorizontal. The strand is then conveyed to a severing apparatus where itis severed into slabs, blooms, billets or other products. The slab orother product then enters a reheat furnace for heating to a uniformtemperature suitable for downstream rolling and other processing.

It has been widely recognized that it is advantageous to directly chargeslabs coming from the caster into the reheat furnace in order to reducethe energy cost associated with reheating slabs that have been cooled toambient temperature.

In general, problems encountered with plate steel product produced bysuch continuous casting mills include the tendency for areas around oneor more surfaces of the steel product to exhibit brittleness, cracking,sponging, and other surface defects. Surface defects are especiallyprevalent after the interim steel product is subjected to downstreamrolling or other stresses. Although the causes of such surface defectsare not completely understood, it has been observed that surface defectstend to occur frequently in steel products having surfaces that are ator above the steel's austenite-to-ferrite transformation starttemperature when the product exits the caster assembly, cool to atemperature above the steel's austenite-to-ferrite transformationcompletion temperature as the product enters the reheat furnace, thenare reheated to a temperature above the transformation start temperaturewhen the product is inside the reheat furnace. Steel products that tendto be particularly susceptible to surface defects include low- tohigh-carbon steels and low-alloy steels, all of which may containaluminum (Al) and residual elements such as sulphur (S), phosphorus (P),nitrogen (N), and copper (Cu).

While an understanding of the causes of the surface defects is not perse necessary for the practice of the invention, some discussion of theapplicant's understanding of the phenomenon may be helpful to thereader. Steel product exiting the caster assembly has a coarse austenitegrain structure. As the steel product cools to a temperature above thetransformation completion temperature of the metal, various elementsincluding residual elements migrate to the austenite grain boundarieswhere they will reside as solute elements, or eventually combine to formprecipitates. If the steel product has not cooled to below thetransformation completion temperature before reheating in the reheatfurnace, these elements, in either solute or precipitate form, remain ator near the original austenite grain boundaries. The presence of theseelements on grain boundaries and/or the development of precipitate-freezones adjacent to grain boundaries can be detrimental to the ductilityof the steel product and may also contribute to the manifestation of oneor more types of surface defects. It appears that the principal culpritin many cases is the copper present.

If the interim steel product is taken off-line and left for severalhours to cool slowly in still air, the entire product will havecompletely transformed from coarse-grained austenite to othermicroconstituents, such as ferrite or pearlite. Reheating this productin a reheat furnace to above the transformation start temperature,(about 900 C. for most steels of interest) the critical temperatureabove which there is austenite recrystallization, re-transforms theproduct into fine-grained austenite. It has been found that a producthaving such a fine-grained austenitic microstructure tends to be freefrom surface defects. However, such slow cooling requires the product tobe taken off-line for an undesirably lengthy period of time, therebyslowing down steel production.

It has been found that instead of re-transforming the entire steelproduct into fine-grained austenite, it is necessary to re-transformonly the surface layers to a suitable depth to achieve a product that isfor the most part free of surface defects. However, off-line slow aircooling to achieve a re-transformed layer of sufficient depth requiresan undesirably lengthy time.

Previously known methods have been devised in which a slab is takenoff-line, immersion-quenched in a quench tank, then returned on-line fortransfer into the reheat furnace. In such methods, the temperature ofthe slab surfaces is often reduced below the transformation completiontemperature, i.e. the steel's transformation completion temperature,before the slab is reheated in the reheat furnace. It has been foundthat an immersion-quenched slab tends to exhibit undesirablyinconsistent metallurgical properties along its length. Thisinconsistency appears to be due to the formation of a lengthwisetemperature gradient on the slab prior to its immersion; since the slabis cast from a continuous caster, its downstream portions have had moretime to cool than its upstream portions.

One way to reduce the surface temperature, is to quench the surface ofthe slab with water as it exits the caster. Quenching processes aredisclosed in such prior art references as U.S. Pat. No. 5,915,457; U.S.Pat. No. 6,374,901 B1; and U.S. Pat. No. 6,557,622 B2, the completedisclosures of each of which are hereby expressly incorporated herein byreference. The latter two references in particular being directed tomethods for differential quenching of the surface of the slabs toaddress the transverse temperature profile of the slab surfaces.

However, there is a disadvantage to water quenching, in that the castingspeed must be restricted to ensure a sufficient depth of the slab hasbeen cooled below a critical temperature, which can negatively impactproductivity. Furthermore, certain grades of steel, are susceptible tocracking as a result of the quench, notably vanadium-bearing steels.

It would be desirable to provide a continuous casting process whichenjoys the benefits of hot-charging the cut slabs into the reheatfurnace to save on energy costs, while avoiding the risks of surfacecracking that are associated with water quenching.

This and other desirable characteristics of the invention will becomeapparent in view of the present specification, including claims, anddrawings.

SUMMARY OF THE INVENTION

The present invention comprises, in part, a method for making steelslabs, comprising the steps of:

casting a continuous steel strip in a caster;

severing the continuous strip into discrete slabs;

directing the discrete slabs, successively, to a holding facility;

holding each of the slabs in the holding facility, and releasing them,successively, while exposing the slabs to ambient air temperature; and

conveying the slabs successively to a reheat furnace;

the slabs being successively held in and released from the holdingfacility a sufficient amount of time to cool surface layers of therespective slabs, to a selected depth of penetration so as to transformaustenite in the surface of the casting to a non-austeniticmicrostructure.

The step of holding each of the slabs in the holding facility, andreleasing them, successively, while exposing the slabs to ambient airtemperature preferably comprises the step of:

moving the slabs along a conveyor path having sufficient length, and ata predetermined speed, such that each of the slabs has attained adesired maximum surface temperature at the time of arrival of thatrespective slab to the reheat furnace.

The method preferably further comprises the step of affirmatively movingambient air across the slabs while the slabs are being moved along theconveyor path.

The present invention also comprises a method for making steel slabs,comprising the steps of:

casting a continuous steel strip in a caster;

severing the continuous strip into discrete slabs;

directing the discrete slabs, successively, to a holding facility;

holding each of the slabs in the holding facility, and releasing them,successively, while exposing the slabs to ambient air temperature; and

conveying the slabs successively to a reheat furnace;

the slabs being successively held in and released from the holdingfacility a sufficient amount of time to cool surface layers of therespective slabs, to a maximum temperature of 1200° F.

The present invention also comprises an apparatus for making steelslabs, comprising:

a caster for casting a continuous steel strip;

a severing device for the continuous strip into discrete slabs;

a conveyor for directing the discrete slabs, successively, to a holdingfacility;

a holding facility for holding each of the slabs in the holdingfacility, and releasing them, successively, while exposing the slabs toambient air temperature;

a conveyor for conveying the slabs successively to a reheat furnace;

the slabs being successively held in and released from the holdingfacility a sufficient amount of time to cool surface layers of therespective slabs, to a selected depth of penetration so as to transformaustenite in the surface of the casting to a non-austeniticmicrostructure.

The holding facility preferably is a conveyor path having sufficientlength, for moving the slabs at a predetermined speed, such that each ofthe slabs has attained a desired maximum surface temperature at the timeof arrival of that respective slab to the reheat furnace.

The apparatus preferably further comprises at least one air movingapparatus, operably configured to create a flow of moving ambient air,over the slabs as the slabs move along the conveyor path.

The invention further comprises a method for making steel slabs,comprising the steps of:

casting a continuous steel strip in a caster;

severing the continuous strip into discrete slabs;

directing the discrete slabs, successively, to a holding facility;

holding each of the slabs in the holding facility, and releasing them,successively, while exposing the slabs to ambient air temperature; and

conveying the slabs successively to a reheat furnace;

the slabs being successively held in and released from the holdingfacility a sufficient amount of time to cool surface layers of therespective slabs, to a selected depth of penetration so as tosubstantially preclude the formation of surface defects in the surfaceof the casting upon placement of the still substantially above ambienttemperature slabs into the reheat furnace.

The invention also comprises steel slabs, made by a method for makingsteel slabs, the method comprising the steps of:

casting a continuous steel strip in a caster;

severing the continuous strip into discrete slabs;

directing the discrete slabs, successively, to a holding facility;

holding each of the slabs in the holding facility, and releasing them,successively, while exposing the slabs to ambient air temperature; and

conveying the slabs successively to a reheat furnace;

the slabs being successively held in and released from the holdingfacility a sufficient amount of time to cool surface layers of therespective slabs, to a selected depth of penetration so as to transformaustenite in the surface of the casting to a non-austeniticmicrostructure.

Preferably, the step of holding each of the slabs in the holdingfacility, and releasing them, successively, while exposing the slabs toambient air temperature further comprises the step of:

moving the slabs along a conveyor path having sufficient length, and ata predetermined speed, such that each of the slabs has attained adesired maximum surface temperature at the time of arrival of thatrespective slab to the reheat furnace.

The invention also comprises steel slabs, made by a method for makingsteel slabs, the method comprising the steps of:

casting a continuous steel strip in a caster;

severing the continuous strip into discrete slabs;

directing the discrete slabs, successively, to a holding facility;

holding each of the slabs in the holding facility, and releasing them,successively, while exposing the slabs to ambient air temperature; and

conveying the slabs successively to a reheat furnace; the slabs beingsuccessively held in and released from the holding facility a sufficientamount of time to cool surface layers of the respective slabs, to amaximum temperature of 1200° F.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a portion of a continuouscasting line, including a water quench apparatus, according to a priorart method and apparatus.

FIG. 2 is a schematic interior side elevation fragment view of anembodiment of the quench apparatus according to the prior art method andapparatus of FIG. 1.

FIG. 3 is a schematic illustration of an apparatus for air-cooling ofslabs, for using the method of the present invention.

FIG. 4 is a more detailed illustration of portions of the apparatus ofFIG. 3, focusing on the slab “walk-around” conveyor system for achievingair-cooling.

DETAILED DESCRIPTION OF DRAWINGS

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail, a preferred embodiment with the understanding that the presentdisclosure should be considered as an exemplification of the principlesof the invention and is not intended to limit the invention to theembodiment so illustrated.

FIGS. 1-2 illustrate a prior art continuous casting facility, in whichwater quenching is employed. A portion of a casting line of a continuouscasting steel facility in which a quench apparatus 12 was installed, isschematically illustrated in FIG. 1. Typically, molten steel was pouredfrom a ladle 14 into a tundish 16 that acts as a temporary reservoir.The molten steel was poured from tundish 16 into a mold 18, which waswater cooled so that the surface of the steel passing through the mold18 solidified to form a continuous thin-skinned strand 19. The strand 19exited the mold 18 and entered a strand containment and straighteningapparatus 20 in which it continued to solidify as it continued to cool,moved arcuately from a generally vertical orientation to a generallyhorizontal orientation, and was straightened to a horizontalorientation. The devices just described collectively constituted acaster assembly 21.

Referring to FIG. 2, after exiting the caster assembly 21, the strand 19was conveyed along the conveyor line at the caster speed by a pluralityof spaced conveyor rolls (table rolls) 22 (generally, this stage may bereferred to as “caster run-out”) and was fed into the quench apparatus12 through a quench apparatus entrance port 23. In this embodiment, thequench apparatus 12 was located immediately downstream of the casterassembly 21 and upstream of a strand severing apparatus 25 (FIG. 1). Thequench apparatus 12 had a housing surrounding the strand 19 andconfining the quench spray. The strand 19 after being quenched exitedthe housing via an exit port.

When the strand 19 was conveyed into the quench apparatus 12, selectedportions of the strands were quenched by a plurality of intense spraysof water and air combined into an air mist applied by clusters of topspray nozzles 31 and bottom spray nozzles 24. (Air mist tends to be moreefficient than water to quench steel.) As a result of the quench, thesteel was rapidly cooled from its pre-quench start temperature to asuitable completion temperature so that the steel's microstructure waschanged from austenite to one or more suitable microconstituents, suchas ferrite or pearlite. Effecting a surface quench to a suitable depth,then reheating the steel in a reheat furnace 29 downstream of thesevering apparatus 25, reduced or prevented altogether the occurrence ofthe surface defects in the steel product. Suitable transformedmicrostructures include pearlite, bainite, martensite and ferrite, orsome combination of two or more of these.

The preferred start temperature was at or above the steel'stransformation start temperature and the suitable completion temperaturewas at or below the steel's transformation completion temperature.Quenching from a start temperature below the transformation starttemperature and above the transformation completion temperature was insome cases acceptable but not preferred, as quenching in thistemperature range provided some but not as much reduction in theoccurrence of surface defects as quenching from a temperature above thetransformation start temperature.

The steel transformation start and completion temperatures depended onthe type of steel that is cast and the cooling rate. Most types of steelcast in a conventional continuous casting mill were deemed suitable forwater quenching; for example, typical plain carbon steels suitable forquenching included steels having 0.03-0.2% carbon content. The coolingrate of a steel product was not constant throughout its body; coolingrates differ at different depths beneath the product surface. Differentcooling rates transformed austenite to different combinations oftransformation products; as the steel's cooling rate varied with stranddepth, the transformed microstructure differed with strand depth. Aminimum transformed depth of about ½ to ¾ inch was deemed tosatisfactorily reduce the occurrence of surface defects.

The spray nozzle clusters 31, 24 were respectively arranged into a toparray 26 and a bottom array 28, wherein each array 26, 28 appliedcooling spray to an associated top and bottom surface of the strand 19.Each array 26, 28 was longitudinally aligned and had a series oflongitudinal banks 26, 28 arrayed in parallel so as to provide spraycoverage to the entirety of the top and bottom surfaces of amaximum-width strand 19.

The appropriate proportions of cooling fluid that was deemed appropriateto be applied respectively to the top and bottom surfaces so that bothsurfaces were quenched to the same depth were said to be empiricallydetermined by removing test portions of the quenched strand andexamining their cross-section. The appropriate proportion could then beprogrammed into a control system for the quench so that subsequentlyquenched portions of the strand would be quenched to the required depth.

Referring back to FIG. 1, after the strand 19 had been quenched by thesprays of the quench apparatus 12, the strand 19 exited the quenchapparatus 12 and was severed into slabs by the severing apparatus 25.The slabs were then conveyed into the reheat furnace 29, where thequenched portions of the slab were reheated to a temperature at least orabove the steel's transformation start temperature, therebyre-transforming the transformed microstructure into austenite. Inpractice, the slabs were heated beyond the transformation starttemperature, to provide a suitable temperature for controlled downstreamrolling. It had been found that the austenite formed by this combinationof quenching and reheating tended to have a finer grain size thanaustenite grains of a steel product that had not been quenched beforereheating. It had further been found that formation of finer grains ofaustenite were associated with the reduction in the occurrence ofdefects in the surface of the eventual steel product.

After quenching, the product was passed into a reheat furnace, where itwas heated to a temperature suitable for subsequent downstreamprocessing. In the reheat furnace, each quenched surface layer wasreheated to a temperature above the transformation beginning temperatureand re-transformed to finer grains of austenite, thereby reducing theoccurrence of surface defects on the eventual steel plate product.

However, as indicated hereinabove, this prior art water quenchingprocess and apparatus has some potential drawbacks, in terms of generalproductivity, as well as potential cracking issues, for cracking-pronegrades of steel (which are well-known to those of ordinary skill in theart of steel-making), such as vanadium-bearing grades.

To address those issues, the present invention is directed to analternative method for cooling the slabs, in order to lower theirsurface temperature to a sufficient depth, to resist cracking, whilestill enabling hot-charging of the slabs into the reheat furnace.

In particular, vanadium-bearing steels are not water quenched, due tothe particularly slow rate of quenching required to prevent thecrack-prone material from developing surface cracks during the quench.Instead, vanadium bearing steels are typically allowed to air-cool fortwenty-four hours, once cut into slabs, prior to charging to the reheatfurnace. This process requires substantially more heat energy to beexpended, to bring the substantially cooled slabs up to the desiredreheat temperature, as compared to the water quench and directhot-charging process described with respect to FIGS. 1-2 herein.

It has been determined that brief cooling the slabs with ambienttemperature air can enable hot-charging of the reheat furnace, withoutrequiring the expensive and complex water quench system, of the priorart, as described and shown in FIGS. 1-2.

The method and apparatus of the present invention, is illustratedschematically in FIG. 3, and in further detail in FIG. 4, whichoverlaps, to some extent, FIG. 3. The method of the present inventionincorporates a ladle 14′ into a tundish 16′ that acts as a temporaryreservoir. The molten steel is poured from tundish 16′ into a mold 18′,which is water cooled so that the surface of the steel passing throughthe mold 18′ solidifies to form a continuous thin-skinned strand 19′.The strand 19′ exits the mold 18′ and enters a strand containment andstraightening apparatus 20′ in which it continues to solidify as itcontinues to cool, moves arcuately from a generally vertical orientationto a generally horizontal orientation, and is straightened in itshorizontal orientation. The devices just described collectivelyconstitute a caster assembly 21′. The strand 19′ is then severed bysevering apparatus 25′ (which may be a mechanical cutting device or aflame cutting device, or other suitable severing device known to thoseof ordinary skill in the art). The portion of the path immediatelydownstream of the caster assembly 21′ may be referred to as the casterrun-out.

After the strand 19′ is severed into slabs 100, they are transported byroller conveyor 22′, to a holding facility 102 (see FIG. 4), which issized sufficiently to absorb slabs 100, e.g., in a planar loop or loops,so that the slabs 100 are maintained in a preferably slowly continuouslymoving queue, to successively hold, and release the slabs 100 at a ratethat will enable the incoming slabs 100 to be taken in, without “backingup” and slowing the casting rate, while at the same time permitting theslabs to cool sufficiently to be returned to downstream conveyor 104,and thence on to reheat furnace 106, and then to subsequent processing,as desired.

The shape, construction and configuration of the holding facility 102may be of any suitable format. In prior art casting installations,typically slabs have been taken “off-line” to a slab yard, forlong-duration holding of slabs (such as for the twenty-four hour holdingdescribed above) and sometimes stacked, because there was insufficientfloor space to enable the slabs to be spread out.

Holding facility 102, in an embodiment of the invention, may compriseone or more conveyor paths, such as paths A (most preferred) and B ofFIG. 4. Note that any numerical physical dimensions provided in FIG. 4are given by way of example, and the invention is not intended to belimited thereby. Holding facility 102 includes two paths which may beused in combination, for example where slabs may be, for short periodsof time, precluded from leaving holding facility 102, because oftemporary lack of availability of reheat furnace 106 or other delayingfactors. In addition, holding facility 102 does contain the option ofallowing slabs to be moved to the slab yard, for exceptionalcircumstances, such as the observation of surface defects on the slabs,extended unavailability of downstream facilities, etc.

Preferably, the casting facility, and more particularly, the holdingfacility, will be situated so that the air temperatures ambient to theholding facility will be, on average, 20° C. to 25° C. Because thetypical casting facility will be constructed to be exposed to outsideair through large doors, the air temperatures in the vicinity of theholding facility may have seasonal variations, from 0° C. in winter to45° C.-50° C. in summer (depending also upon the geographic location ofthe casting facility in general).

A typical casting strand may have an initial thickness of 6 inches,which is subsequently rolled down to 1.25-1.5 inches in thickness, andwill have a more or less constant width, during rolling, ofapproximately 73 inches. These dimensions are given by way of example,and the scope of the invention is in no way intended to be limitedthereto. At the time and location of the cut (e.g., by torch) intoslabs, each slab may have surface temperatures of from about 1550° F. toabout 1900° F. or greater, and a substantially higher core temperature.The temperatures coming off the initial casting are, as indicatedhereinabove, highly non-uniform throughout the surface and volume ofeach slab. It is believed for a slab of such characteristics, that itshould be cooled to a surface temperature of 1200° F. or less (e.g.,1150° F. to provide a further margin of safety) to have cooling to asufficient depth to enable hot direct charging to the reheat furnacewhile substantially eliminating surface cracking or other surfacedefects.

It has been found that a cooling time for such slabs, when placed in aholding facility can be in the range of 60 to 90 minutes, and achievethe desired degree of surface cooling, even with a casting rate of 50inches per minute. Increases in initial temperatures will, of necessity,increase the time that the slabs must loiter in the holding facilitybefore being moved on to the reheat furnace. Those of ordinary skill inthe art can determine, from simple calibration tests, the requiredincrease in holding time, or less preferably, reduction in castingspeed, which would affect the desired cooling. Conversely, reductions inthe temperatures of the slabs reduces the holding time between slab cutand reheat furnace charge. Variations in slab dimensions, from thosediscussed, may also have some effect on cooling times, though not aslarge an effect as variations in initial slab temperatures, inasmuch asit is the achievement of necessary cooling to a particular depth whichis important (which may be more or less independent of the overallthickness of the slab being cooled).

As mentioned above, a preferred embodiment of the method incorporatesthe use of a planar track comprising one or more loops or alternativepaths, to hold the slabs during their ambient air cooling period.

The holding facility may also be provided with one or more air fans(e.g., fan 110), or other air conditioning devices, which may be placedat various locations around the path of the slabs in holding facility102, to keep the air moving around the slabs in the holding facility.This is particularly beneficial, in that it is well known that in metalcasting installations, in the absence of devices to move air along, airtemperatures can be substantially elevated.

Although stacking of the slabs might be considered as an alternativemeans for enabling the slabs to loiter in between the caster run-out andthe reheat furnace, in general, stacking is considerably less preferred,as the proximity of the slabs to one another tends to slow the rate ofcooling, making the air-cooling process less efficient. A stackingstructure might be incorporated into the overall conveyor system forpermitting stacking of the slabs, for brief periods of time, in theevent of rare occasional occurrences when the reheat furnace might notbe clear to receive cooled slabs.

The foregoing description and drawings merely explain and illustrate theinvention, and the invention is not so limited as those skilled in theart who have the disclosure before them will be able to makemodifications and variations therein without departing from the scope ofthe invention.

1. A method for making steel slabs, comprising the steps of: casting acontinuous steel strip in a caster; severing the continuous strip intodiscrete slabs, each of the slabs having a surface temperature in excessof the austenite-to-ferrite transformation completion temperature of thesteel; moving the slabs from the caster to a reheat furnace along atleast one continuous conveyor path from amongst a plurality ofalternative continuous conveyor paths, all of which lead to the reheatfurnace and convey the slabs from the caster to the reheat furnace,without removing the slabs from the conveyor path, while exposing theslabs to ambient air temperature, the at least one conveyor path havinga sufficient length and predetermined speed to ensure that only thesurface layers of the respective slabs cool enough through a selecteddepth of penetration to transform from an austenite microstructure, toensure that substantially less than the entirety of each of therespective slabs transforms from its austenite microstructure prior tointroduction into the reheat furnace, the surface layers of therespective slabs cooling to a maximum temperature of about 1200° F. as aresult of said movement along said at least one continuous conveyorpath; and conveying the slabs successively to and into the reheatfurnace.
 2. The method according to claim 1 wherein the length and thepredetermined speed of the at least one conveyor path are such that onlythe surface layers of the respective slabs cool to a temperature ofabout 1150° F.
 3. The method according to claim 1, further comprisingthe step of affirmatively moving ambient air across the slabs while theslabs are being moved along the at least one conveyor path.